A closer look at how propellers cause forward thrust will reveal that the
hovercraft moves forward by pushing air behind it. Exactly how does the
propeller push air behind it? To understand this we turn to a principle that
was discovered about 300 years ago, Bernoulli’s Principle.

Daniel Bernoulli
1700 – 1782

Bernoulli’s Principle: An increase in the velocity of
any fluid is always accompanied by a decrease in pressure.

Since air behaves exactly like a fluid, Bernoulli’s principle applies. Any
time the wind is blowing or a fan blows air, the pressure of the moving air
becomes less than it would be if the air was not moving. As an aside, this
characteristic plays a huge role in how weather systems work! If you can cause
air to move faster on one side of a surface than the other, the pressure on
that side of the surface will be less than the pressure on its other side.

One of the most widely used applications of Bernoulli's principle is in the
airplane wing. Wings are shaped so that the top side of the wing is curved
while the bottom side is relatively flat. In motion, the front edge of the wing
hits the air, and some of the air moves downward below the wing, while some moves
upward over the top. Since the top of the wing is curved, the air above the
wing must move up and down to follow the curve around the wing, while the air
below the wing moves very little. The air moving on the top of the curved wing
must travel faster than the air moving under the wing. The air pressure on
the top of the wing is therefore less
than that on the bottom of the wing, according to Bernoulli’s principle. The
higher-pressure air on the bottom of the wing pushes up on the wing with more
force than the lower-pressure air above the wing pushes down. This results in a
net force acting upwards called lift.
Lift pushes the wings upward and keeps the airplane in the air.

While Bernoulli's principle is a major source of lift in an aircraft wing,
a Romanian engineer by the name of Henri Coanda discovered another effect that
plays an even larger role in producing lift.

Henri Coanda
1886 – 1972

Although generally unrecognized, Coanda was actually the first person to
build and fly a jet powered aircraft. It is commonly believed that the first
jet engines were developed during World War II. Dr. Hans Von Ohain designed the
first German jet aircraft, which made its first flight on August 27, 1939.
Unaware of Dr. Von Ohain's work, A British engineer named Sir Frank Whittle also
independently designed a jet aircraft, which first flew on May 15, 1941.

Although these two men are generally thought of as the fathers of jet aircraft,
Henri Coanda built and "flew" the first recorded jet aircraft about
30 years earlier. The somewhat amusing first flight is best described in Coanda's own words:

“It was on 16 December 1910. I had no intention of flying on that day.
My plan was to check the operation of the engine on the ground but the heat of
the jet blast coming back at me was greater than I expected and I was worried
in case I set the aeroplane on fire. For this reason
I concentrated on adjusting the jet and did not realize that the aircraft was
rapidly gaining speed. Then I looked up and saw the walls of Paris
approaching rapidly. There was no time to stop or turn round and I decided to
try and fly instead. Unfortunately I had no experience of flying and was not
used to the controls of the aeroplane. The aeroplane seemed to make a sudden steep climb and then
landed with a bump. First the left wing hit the ground and then the aircraft
crumpled up. I was not strapped in and so was fortunately thrown clear of the burning
machine.”

Unfortunately Coanda couldn’t obtain funding to continue his research after
the wreck, so his contribution to jet propulsion never became widespread.
If he had been able to continue his work, France
could have had a jet-powered air force before WWII. Even though he
didn't build another jet aircraft, Coanda did make an interesting observation, which
led to a very important contribution to the knowledge of how aircraft wings
produce lift. As the plane was flying, he noticed that the flames and exhaust
from the engine were flowing along the edges of the aircraft. This phenomenon
is now called the Coanda Effect.

Coanda Effect: A moving stream of
fluid in contact with a curved surface will tend to follow the curvature of the
surface rather than continue traveling in a straight line.

To perform a simple demonstration of this effect, grab a spoon and find a
sink. Get a small stream of water coming down from the sink, then
place the bottom of the spoon next to the stream. Notice how the water curves
along the surface of the spoon. If you hold the spoon so that it is free to
swing, you should be able to notice that the spoon is actually being pulled
towards the stream of water.

The same effect occurs with an airplane wing. If the wing is curved, the
airflow will follow the curvature of the wing. In order to use this to produce
lift, we need to understand the term angle of attack,
which refers to the angle between the wing and the direction of the air flow, as
shown in the following diagram.

The angle of attack indicates the degree to which the wing is tilted with respect to the
oncoming air. In order to produce lift, or an upward
force acting on the wing, Newton's
Third Law says that there must be an equal force acting in the opposite
direction. If we can exert a force on the air so that it is directed down, the
air will exert an upward force back on the wing. Figure 17-5 illustrates how
the Coanda Effect directs the airflow for varying angles of attack:

This diagram shows that increasing the angle of attack increases how much
the air is deflected downwards. If the angle of attack is too great, the air
flow will no longer follow the curve of the wing. As shown in the bottom of the
diagram, this creates a small vacuum just behind the wing. As the air rushes in
to fill this space, a process referred to as cavitation, it causes
heavy vibrations on the wing and greatly decreases the efficiency of the wing.
For this reason, aircraft wings are generally angled like the middle wing in
the diagram. This wing efficiently directs the airflow downward, which in turn
pushes up on the wing, producing lift.

This method of determining lift is called momentum change. Other methods to
calculate the same lift utilize the difference in pressure fields above and
below the wing. Either method, using momentum or using pressure, is accurate
on its own, but never use the two methods together. To do so could make the
lift appear to be twice its actual value.

In addition to producing lift on an aircraft, Bernoulli's Principle and the
Coanda Effect play an important role in the operation of a propeller. Examine a
propeller closely and you will find that the blades of the propeller look like
an airfoil, or wing. Essentially, a propeller blade is a wing turned on its
side. Just as wings traveling forward are lifted upward, a rotating propeller
blade is sucked or pushed forward. Propeller blades, however, have one feature that
is lacking in wings: they are twisted. Watch a propeller turn very slowly, and you will
see how the twist of the blade allows it to move the air evenly and push it
backward. Additionally, propeller blades are set at an angle, referred
to as propeller pitch. The greater the pitch of the propeller,
the more air it can push. Blades of common household fans are also slightly
angled to help move air for cooling. Ideally, an equal quantity of air will pass
the blade at both its root (the hub of the propeller) and at its tip, but the tip
travels much faster than the root. To maintain a flow rate that is as even as
possible, the hub pitch (pitch at the root of the propeller) must be very
steep while the propeller tips have to be almost flat! This helps to ensure an
even flow of air through the duct.